all areas can be studied by a family physician, such as chronic diseases, mental disorders among elder people.

No limit. Family physicians should know everyting about everything.

There must a limit in my opinion. The question may be changed as 'in which studies a family physician would be the leader of the study team (the first author)?'

I think family physicians (called general practitioners in the UK) need to have a reasonable working knowledge of all patient-facing specialties, and a good understanding of the role of the non-patient-facing specialties (e.g. radiology, pathology, microbiology etc.) and when to seek their advice.

It's really difficult to know the depth of knowledge required in all specialties, but I don't think it would be fair (or indeed realistic) to expect family physicians to have the same knowledge of specialists in those constituent areas (as of course specialists generally are focused on their specific area of specialism, though of course even in their case it's good to maintain a reasonable working knowledge of general medicine).

I think it's really important the family physicians are aware of the red flags for serious illnesses - even if they don't know the treatment in depth. Just because you rely on your family physician to pick up on early signs of malignancies, for example.

In my opinion, strict limitation is impossible, even meaningless. Maybe rotation departments of family medicine speciality education may give an idea, but on the other hand; the patient spectrum and personal interests in family medicine make such requests meaningless. I believe that the variety of articles in the leading journals of family medicine can be used when defending the discipline.

I personally plan researches on what I want to find an answer for any question in my mind. However, I suggest for my family medicine residency assistants to study on subjects they meet at primary care offices.